Direct-current-coupled transistor power amplifiers



M G. REIFFIN April 2, 1968 3,376,388 mREcT-cURRENflcoUPLED TRANSISTOR POWL-R AMPLIFIERS I 9 Sheets-Sheet 1 Filed May 10, 1967 INVENTOR.

April 1968 M. G. REIFFIN 3,376,388

ISTOR POWER AMPLIFIER DIRECT-CURRENT-COUPLED TRANS 9 Sheets-Sheet Filed May 10, 1967 INVENTOR MARTIN G. REIFFIN M. G. REIFFIN A ril 2, 1968 DIRECTCURRENTCOUPLED TRANSISTOR POWER AMPLIFIERS 9 Sheets-Sheet Filed May 10, 1967 April 2, 1968 M. G. REIFFIN 3,376,388

DIRECT-CURRENTCOUPLED TRANSISTOR POWER AMPLIFIERS Filed May 10, 1967 9 Shets-Sheet 4 FIG. 2a FlG.3a

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PRIOR ART DIRECT-CURRENT-COUPLED TRANSISTOR POWER AMPLIFIERS Filed May 10, 1967 9 Sheets-Sheet L we 7; [7 /?45 I28 I23 5 Q f a 72 INVEN TOR.

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M. G. REIFFIN April 2, 1968 DIRECT-CURRENT-COUPLED TRANSISTOR POWER AMPLIFIERS 9 Sheets-Sheet 3 Filed May 10, 1967 mmm mmm

Sm mH vmu April 2, 1968 M. G. REIFFIN 3,376,338

DIRECT-CURRENT-COUPLED TRANSISTOR POWER EXMPLIFIHRS Filed May 10, 1967 9 Sheets-Sheet 7 INVENTOR.

April 1968 M. G. REIFFIN 3,376,388

DIRECT-CURRENT-COUPLED TRANSISTOR POWER AMPLIFIERS 3,376,388 DIRECT-CURRENT-COUPLED TRANSESTOR POWER AMPLHFIERS Martin G. Reiflin, 1.02 Gallows Hill Road, Peekskill, N.Y. 10566 Continuation-impart of applications Ser. No. 311,732,

Sept. 26, 1963, Ser. No. 388,399, Aug. 10, 1964, and Ser. No. 563,586, July 7, 1966. This application May 10, 1967, Ser. No. 654,947

51 Qlaims. (Cl. 179--1) ABSTRACT OF THE DISCLOSURE A transistor power amplifier is provided with directcurrent-coupling between stages and to the loudspeaker. The output stage is push-pull single-ended and includes two transistors connected in series at a midpoint of the stage. A split power supply has a center-tap connected to one output terminal and the other output terminal is connected to said output stage midpoint. Amplification and drive stages are direct-current-coupled in cascade to the output stage. A direct-current negative feedback network extends from the output stage to an amplification stage to maintain the output terminals at substantially the same quiescent potential to prevent direct-current flow through the voice-coil of the loudspeaker.

Cross-References This application is related to my prior applications listed below:

Transistor Power Amplifiers, Ser. No. 270,011, now Patent No. 3,281,535, filed Apr. 2, 1963; Transistor Power Amplifiers and Feedback Speaker Systems Embodying Same, Ser. No. 311,732, filed Sept. 26, 1963; Transistor Power Amplifiers and Feedback Systems Embodying Same, Ser. No. 388,399, filed Aug. 10, 1964; Transistor Power Amplifiers for High Fidelity Music Reproduction, Ser. No. 563,586, filed July 7, 1966.

The present application is a continuation-in-part of said applications Ser. No. 311,732; Ser. No. 388,399 now abandoned; and Ser. No. 563,586. Said application Ser. No. 388,399 is in turn a continuation-in-part of said application Ser. No. 270,011 now Patent No. 3,281,535.

Field of the invention tions where high power output and inaudible amplifier distortion under rigorously critical listening conditions are required.

Description 09 the prior art In the prior state of the art, the advent of transistors, with their ability to function efiiciently when loaded by the relatively low impedance of a loudspeaker, made feasible the elimination of the output transformer from audio amplifiers. Since the output transformer was by far the most expensive component and generated the most distortion, phase shift and feedback instability, its elimination would have provided a substantial advance if it were not for the fact that prior transistor amplifier circuits substituted other components which were almost as disadvantageous.

More specifically, one widely adopted design approach provides direct coupling between the output stage and the loudspeaker, but utilizes a driver transformer to drive the output transistors. The primary of the driver transformer is capacitor-coupled to ground. Another prior art solution involves direct coupling the drive stage to the output stage but coupling the latter to the loudspeaker through 3 ,376,388 Patented Apr. 2, 1968 a large electrolytic capacitor. Both of these circuit types frequently employ interstage coupling capacitors as well.

Instead of utilizing these reactive coupling components, the amplifier circuit of the present invention provides direct coupling between the several stages, to the loudspeaker, and throughout the feedback loop. In addition to the obvious advantages of reduced cost and greater reliability due to the elimination of transformers and coupling capacitors, the primary advantage of the present direct-coupled circuit resides in the elimination or substantial reduction of the following forms of distortion which are generated by or inadequately reduced by the conventional circuits:

(1) Nonlinear dist0rti0n.-Driver transformers and coupling capacitors produce phase shift at both frequency extremes so as to prevent the application of a large amount of feedback with stability. Nonlinear distortion, including both harmonic and intermodulation distortion due to nonlinear transistor transfer characteristics, is reduced in an amount proportional to the amount of feedback. The restriction to a moderate amount of feedback, necessary for stability in the conventional circuits, results in a substantial residuum of nonlinear distortion.

(2) Oscillatory resp0nse.At low frequencies this is known as breathing and at high frequencies as ringing. In either case, the complex poles of the transfer function provide a spurious oscillatory response if the amount of feedback is excessive in view of the phase shifts provided by the reactive coupling components of the conventional circuits. The designer of the latter is faced with the dilemma of risking this form of distortion or settling for a substantial amount of nonlinear distortion by utilizing only a moderate amount of feedback to obtain a greater stability margin.

(3) Overload rec0very.At overload peaks when one or more transistors go into saturation, it is usual for at least one interstage coupling capacitor to pass a large unidirectional surge of current so as to change its state of charge. After the overload signal terminates it takes the capacitor a finite time to recover its original charge condition. During this recovery period the bias on the succeeding amplification stage is usually sufiiciently disturbed by the potential applied thereto by the capacitor so as to cause either radical distortion or even complete cut-off known as blocking.

(4) Crossover distortion.-Although inherent in any transistor push-pull output stage operating near the Class B bias mode, this type of distortion is reduced approximately in proportion to the amount of feedback, provided the distortion is not so severe as to reduce the amplifier gain to the point where the feedback is ineffective. As in the case of nonlinear distortion, the phase shifts generatedby the reactive coupling components of the conventional circuits prevent the application with stability of a large amount of feedback and hence the residual crossover distortion is usually excessive.

(5) Transient distortion.Although this term is sometimes applied to various forms of spurious response to nonsinusoidal periodic signals such as square waves, it is intended here to refer to distortion arising from nonperiodic pulse signals. One form produces premature clipping at the rise of a tone burst due to a disturbance in the bias condition of an early stage and produced by a low frequency instability triggered by the burst. Another form occurs during the recovery from a heavy pulse having a substantial direct-current component which changes the charge condition of a coupling capacitor, usually the output capacitor which couples to the speaker rather than an interstage capacitor as in the case of overload recovery distortion.

(6) Bias drift-Varying thermal and load conditions and changes in component parameters may cause a drift of the quiescent operating point of one or more stages, resulting in premature clipping at power levels lower than rated. The use of reactive coupling in the conventional circuits usually prevents the application of a large amount of overall direct-current feedback which would counteract any tendency of the bias point to drift from the optimum value.

(7) Transformer crossover transients.lf the two halves of a driver transformer primary winding are not perfectly coupled, crossover switching transients in the form of notches or ringing will occur, particularly when abetted by carrier storage at high frequencies. Trifilar windings increase the coupling and thus reduce these transients, but do not completely eliminate them.

(8) Reactive coupling loads.The use of an output coupling capacitor results in an elliptical load-line for the amplifier at low frequencies, and the primary inductance of a driver transformer provides a similar reactive load for the drive stage. In response to low-frequency transient signals the operating point of the output stage may enter regions of simultaneously high voltage and current so as to result in second breakdown causing the destruction of the output transistors.

Summary of the invention It is therefore a primary object of the present invention to provide novel transistor power amplifier circuits wherein the pre-drive, drive and output stages are D.C.-coupled to each other and the output stage is D.C.-coupled to the loudspeaker, so as to eliminate the serious disadvantages inherent in the use of driver transformers and output coupling capacitors. Prior art circuits having the last two or three stages D.C.-coupled in this manner have heretofore been unsuccessful because they were deficient in one or more of the following respects:

(1) There was an inability to maintain D.C. balance at the amplifier output terminals under varying ambient temperatures, line voltages and output power loads. Any substantial D.C. unbalance produces a DC. current in the loudspeaker voice-coil thereby biasing the cone off center and increasing the speaker distortion.

(2) The power supply ripple was injected into an early stage of the amplifier thereby producing an excessive amount of hum.

(3) The power and distortion characteristics did not compare favorably with other circuits.

(4) Some of the circuits were complex and critical and therefore costly "and unreliable.

(5) Failure of an output transistor was likely to damage the loudspeaker so as to require factory replacement of the cone and voice-coil at substantial expense.

(6) Although measuring extremely well on distortion meters in laboratory bench tests they generated audible distortion when subjected to music signals in critical listening tests.

It is therefore a further object of the present invention to obviate the above-noted defects heretofore prevalent in D.C.-coupled amplifier circuits.

The attainment of practical and properly functioning circuitry wherein the pre-drive, drive and output stages are D.C.-coupled to each other and to the loudspeaker provides an amplifier having phase shift characteristics which permit the use of new and more effective feedback techniques and impedance matching arrangements.

In one embodiment an inner negative feedback loop raises the high-frequency cutofi of the amplifier to a predetermined frequency, and preceding the inner loop is a preamplifier stage having a high-frequency cutoff substantially lower than said predetermined frequency. An outer negative feedback loop includes both the amplifier and the preamplifier stage. The result is an unusually large amount of feedback without instability.

Other objects and advantages are either inherent in the circuitry disclosed herein or will become apparent to those 4 skilled in the art as the detailed description proceeds in connection with the accompanying drawings wherein:

Description of the drawings FIG. 1 is an amplifier circuit embodying the invention;

FIG. 2 shows a modified form of the invention which uses only a single zener diode and a non-split auxiliary power supply, and which also includes an additional foure transistor drive stage having two transistors driven by the output stage through feedback voltage dividers energized by the latter;

FIG. 3 is a schematic circuit diagram of a double-loop feedback system incorporating an amplifier in accordance with the present invention;

FIG. 4 shows another double-loop feedback system wherein the second preamplifier stage is D.C.-coupled to the amplifier;

FIG. 5 shows two amplifiers each in accordance with the present invention and connected in a stereo mode so as to constitute two independent channels;

FIG. 6 shows the same two amplifiers connected in a parallel mode so as to match the relatively low impedance of a single four-ohm loudspeaker;

FIG. 7 shows the same two amplifiers connected in a series mode to form a full-bridge circuit for driving a single loudspeaker of higher impedance such. as sixteen ohms.

FIG. 8 shows a modified form of the invention wherein a zener diode provides a source of reference potential for i FIG. 4a shows another modified form of the invention.

wherein each common-emitter stage comprises "a symmetrical pair of emitter-coupled transistors; and

FIG. 5a shows a widely-used prior art circuit utilizing an output coupling capacitor.

Detailed Description Referring now to the drawings in more detail and first to FIG. 1 thereof, there is shown an amplifier circuit in accordance with the present invention and comprising a voltage amplification or pre-drive stage including transistor Q1, a complementary symmetry push-pull drive stage including transistors Q2 and Q3, and a pushpull power output stage including transistors Q4 and Q5.

Each stage is D.C.-coupled to the next stage and the, output stage is in turn D.C.-coupled to the loudspeaker S.: The pre-drive transistor Q1 is operated common-emitter in Class A. The drive and output stages operate as emitter-follower Darlington pairs and are preferably biased in Class AB with sufiicient quiescent current to minimize bypassed by a capacitor C2. The collector of transistor Q1 is connected to one end of a diode D2 having its opposite end connected to collector load resistors R5, R6. Capacitor C1 has one end connected to output terminal 01 through leads 15, 16 and its opposite end connected to the junction of resistors R5, R6 to provide bootstrapping in the conventional manner.

The battery symbols B2 and B'2 designate the halfsections of a conventional auxiliary power supply having its centertap grounded as shown. A resistor R6 has one end connected to the positive terminal of the positive half-section B2 of the power supply and its opposite end is connected to the positive end of a zener reference diode Z1. The negative end of the latter is connected to ground. The lower ends of diode D1, resistor R3 and bypass capacitor C2 are connected to the positive end of zener diode D1.

A resistor R7 has one end connected to the negative end of the negative half-section B'2 of the auxiliary power supply and its opposite end is connected to the negative end of a second zener reference diode Z2 having its positive end connected to ground. The upper end of collector load resistor R5 is connected to the negative end of zener diode D2.

The potential of the positive terminal of power supply positive section B2 is more positive than the breakdown voltage of zener diode Z1 so that the latter is maintained in its breakdown state with its positive end at a substantially fixed predetermined voltage with respect to ground. Similarly, the negative terminal of power supply negative section B'2 is maintained at greater negative potential than the breakdown voltage of zener diode Z2 so that the latter is maintained in the breakdown state with its negative end at a fixed predetermined negative voltage with respect to ground. Therefore the base, emitter and collector circuits of transistor Q1 are connected to potential sources at fixed predetermined voltages which remain substantially invariant under different conditions of ambient temperature, line voltage and amplifier load.

The collector of transistor Q1 is D.C.-coupled by lead 11 to the base of the lower NPN transistor Q2 of the complementary drive stage and the upper end of diode D2 is similarly D.C.-coupled by lead 12 to the base of the upper PNP transistor Q3 of said drive stage. Bias resistors R9, R extend from the respective emitters of transistors Q2, Q3 to a common junction connected by lead to the output terminal 01. The collector of the lower NPN transistor Q2 is D.C.-coupled by lead 13 to the base of the lower output transistor Q4 and the emitter of the upper PNP drive transistor Q3 is D.C.- coupled by lead 14 to the base of the upper output transistor Q5.

Resistor R8 has one end connected to the collector of drive transistor Q2 and its opposite end is connected to the positive terminal of the positive half-section B1 of a main split power supply having a negative half-section B1 with a negative terminal connected to the collectors of both upper drive transistor Q3 and upper output transistor Q5. The center-tap of the main power supply B1, B1 is grounded as shown and constitutes the grounded output terminal 02 whereby the speaker S may be connected with its voice-coiled terminals D.C.-coupled to the respective amplifier terminals 01, 02.

The emitter of the lower output transistor Q4 is provided with a bias resistor R11 extending to the positive terminal of the positive power supply section B1 and its collector is connected to output terminal 01. The emitter of upper output transistor Q5 is similarly provided with a bias resistor R12 extending to output terminal 01.

The two signal input terminals are indicated at I1, I2. Input terminal I1 is connected directly to the base of transistor Q1 whereas input terminal I2 is grounded as shown. Transistor Q1 operates as a Class A commonemitter amplifier stage and the potential of its collector varies in response to a signal fed to input terminals I1, 12. Diode D2 provides a temperature-compensated bias for drive transistors Q2, Q3 in the conventional manner.

Due to the small voltage drop across diode D2 the potential of lead 12 will be maintained a fraction of a volt more negative than the collector of transistor Q1 as the collector swings in response to a signal input. The complementary drive stage Q2, Q3 provides both phase inversion and emitter-follower operation. That is, since the emitter of a transistor operating in the active region remains within a fraction of a volt of the base, it will be seen that the potential of the junction of emitter resistors R9, R10, and hence that of the output terminal 01, will be maintained intermediate the respective potentials at the opposite ends of diode D2 as the latter swings up and down with the collector of transistor Q1. Thus the potential of amplifier output terminal 01 will be maintained within a fraction of a volt of the potential of the collector of transistor Q1 so as to provide the low distortion of emitter-follower operation.

In view of the D.C.-coupling of the amplifier to the loudspeaker S, it is imperative that the output terminals 01, 02 be maintained at substantially the same DC potential, both under quiescent condition and at various levels of power output, and irrespective of variations in ambient temperature and line voltage. Any D.C. unbalance between output terminals 01, 02 will cause a direct current to flow through the voice-coil of loudspeaker S. This in turn will cause the cone of loudspeaker S to be biased ofi center. If the direct current and the resultant biasing are substantial, the nonlinear distortion generated by the loudspeaker will be seriously increased.

Output terminal 01 is maintained at substantially the same potential as output terminal 02, that is, at ground potential, by two feedback arrangements. The first comprises the emitter bias resistor R3 of transistor Q1 which is made highly effective by zener diode Z1. The breakdown voltage of zener diode Z1 is selected so as to be substantially larger than the quiescent collector-to-emitter voltage of transistor Q1 so that the extra voltage may be dropped across resistor R3. The resistance of the latter may thus be many times larger than is customary or would be permissible in the absence of zener diode Z1.

For example, in conventional prior art circuits of the complementary-symmetry type wherein the lower ends of resistors R3, R8 and R11 are connected to a common ground, the resistor in the relative position of R3 is generally about a few hundred ohms. Any attempt to increase this resistance substantially will cause a corresponding reduction in the available voltage swing of the collector of Q1 and hence in the maximum power output capability of the amplifier. However, if the breakdown voltage of zener diode Z1 is selected so as to be a predetermined amount (e.g., about ten volts) greater than the voltage of the positive terminal of the main power supply section B1 then resistor R3 may have this voltage difference dropped across it. Hence, for the same emitter current, resistor R3 may be of a magnitude of several thousand ohms, that is, approximately ten times larger than in the conventional prior art circuits, without any reduction in the collector voltage swing or the amplifier maximum power output.

Since resistor R3 provides D.C. feedback which stabilizes the collector current of transistor Q1 and hence the potential of the collector of transistor Q1, by increasing the magnitude of this resistor ten-fold, the feedback is increased by the same amount and hence the stability of the DC. potential of output terminal 01 is increased by a like amount.

The second feedback arrangement for the latter purpose is provided by the base bias circuit including elements D1, R1, R2 and P. The potentiometer P and resistor R2 together with leads 15 and 16 provide a DC. feedback path extending from output terminal 01 to the base of transistor Q1. This feedback is degenerative so that it counteracts any tendency of output terminal 01 to vary in DC. potential. Potentiometer P further provides a convenient arrangement for initially adjusting the poten tial of output terminal 01 so that it is at ground potential and thus balanced with respect to the grounded output terminal 02.

The DC feedback provided by potentiometer P and resistors R1, R2 is aided by zener diode Z1 in two important respects. First, the predetermined invariant potential provided by zener diode D1 constitutes a fixed reference voltage which the feedback arrangement utilizes as a comparison standard to detect and correct errors in the DC. potential of output terminal 01. Second, the potential source arrangement of zener diode D1 provides a large potential drop across the lower base bias resistor R1 and hence the latter may be many times larger than is conventional. Since the amount of DC. feedback is determined by the ratio of the magnitude of resistor R1 to the sum of the magnitudes of resistors R1, R2 and potentiometer P, the greatly increased value of R1 results in a substantially larger amount of DC. feedback and hence greater effectiveness in maintaining the DC. potential output terminal 01 fixed at ground level.

Further precision in this regard is provided by diode D1 which serves as a temperature compensating element to counteract the variations in the base-to-emitter voltage of transistor Q1 with variations in ambient temperature. The temperature compensating operation of diode D2 is well-known in the art and similarly counteracts the variation in base-to-emitter voltage of drive transistors Q2, Q3 and output transistors Q4, Q5 with variations in ambient temperature.

It will be seen that variations in amplifier load will have practically no effect on the DC. balance of output terminals 01, 02. That is, as the load increases the regulation of the main power supply B1, Bl will cause the voltage across its terminals to decrease substantially due to the internal impedance of the supply. The center-tap at 02 will remain at the same potential since it is grounded and the respective potentials at the opposite ends of the power supply will shrink toward ground as the load on the amplifier is increased.

However, the DC. potential of output terminal 01 will remain substantially unaifected for the following reasons. The potential of output terminal 01 is determined by the potential of the collector of transistor Q1. Zener diodes Z1, Z2 maintain the lower end of emitter resistor R3 and the upper end of collector resistor R5 at substantially constant fixed voltages irrespective of variations in the load on the amplifier. Hence the DC. potential of the collector of transistor Q1 remains substantially constant, and thus the DC. potential of output terminal 01 is maintained at ground potential under varying load conditions.

For the same reasons, variations in line voltage will not disturb the D.C. balance of output terminals 01, 02 provided that the line voltage be at least the minimal value required to cause the voltages of the auxiliary power supply sections B2, B'2 to be greater than the respective breakdown voltages of zener diodes Z1, Z2 so that the latter may operate in their proper breakdown regions.

The D.C.-coupling between the three high level stages and between the output stage and the loudspeaker provides several important advantages, the foremost probably being the phase shift characteristics which permit degrees and techniques of feedback not possible with conventional transformer and capacitor coupling arrangements. At low frequencies the D.C.-coupling introduces no phase shift whatever. In addition, the relatively large resistance of emitter resistor R3 further improves the low frequency stability margin in any feedback arrangement by providing substantial degeneration and consequent large gain reduction at extreme low frequencies where the bypass capacitor C2 becomes effectively open.

At high frequencies the large feedback stability margin made possible by the emitter-follower operation and Darlington pair arrangement of the last two stages is further augmented by the complete elimination of stray capacitances due to coupling capacitors and transformers, as well as leakage inductance of the latter. The feedback techniques made available by these greatly improved phase shift characteristics are described hereinbelow.

Another advantage of the D.C.-coupling arrangement resides in the improved overload recovery characteristics due to the absence of any interstage coupling capacitors preceding a high level stage. In prior amplifier circuits having such capacitors, overloading of the stage caused a large current to flow through the capacitor so as to change its voltage. This caused the stage to be improperly biased, and most such amplifiers generated severe distortion or even blocked completely for a short period until the capacitor recovered its normal voltage and the bias resumed its proper value.

Still another advantage of the D.C.-coupling arrangement is the reduction of the tendency to second breakdown of the output transistors. The mechanism of second breakdown is not completely understood but is believed to result from the simultaneous occurrence in the transistor of high instantaneous voltage and high instantaneous current. It is probable that in most instances this condition occurs because of the reactive nature of the load at low frequencies when an output coupling capacitor is employed between the output stage and the loudspeaker.

Still another advantage of the present invention resides in the complete isolation of the ripple of the main power supply B1, Bl from the voltage amplification stage comprising transistor Q1. The auxiliary power supply B2, B'2 has negligible ripple, even under heavy amplifier load, because it supplies a very small current which remains small due to the Class A operation of Q1. Also, any residual ripple is smoothed out by the filtering action of the zener diodes.

A further advantage of the D.C.-coupling resides in the impedance matching of the amplifier to the load. The usual 1,000 microfarad output coupling capacitor has at 20 cycles per second twice the impedance of a four-ohm loudspeaker, and hence the maximum power capability of the amplifier is greatly reduced when driving such a loudspeaker. This defect is entirely obviated by the present D.C.-coupling to the speaker.

Furthermore, the present invention makes feasible a modular arrangement whereby two amplifier sections may be selectably connected in either a parallel mode to match lOW impedance speakers (e.g., four ohms) or in a series.

mode to match high impedance speakers, such as sixteen ohms.

types of breakdown components such as gas tubes, or

ordinary D.C. batteries, or the auxiliary power supply B2, B'2 may be of the regulated type. However, the zener diode arrangement disclosed is simpler, less expensive and more reliable than these other expedients.

Referring now to FIG. 2, there is disclosed another modified form of the invention wherein the pre-drive stage is directly connected to the main power supply and utilizing only a non-split auxiliary power supply and a single zener diode.

In more detail, the amplifier of FIG. 2 comprises a first pre-drive stage including a transistor Qlx operating in the Class A common-emitter mode. The base of transistor Qlx is connected to the input terminal 11x, the

other input terminal 12x being grounded as shown. The base is biased by resistors Rlx, R21: and potentiometer Px. The emitter of transistor Qlx is provided with two bias resistors R3x, R4x. The lower resistor R3x is bypassed by a capacitor Clx whereas the upper resistor R4x is unbypassed, for a purpose to be described. The.

collector of transistor Qlx is connected to the lower end of a load resistor R5x.

A zener diode Z has its positive end connected to ground and its negative end is connected to the upper end of collector load resistor R6x and the upper end of potentiometer Px whereby the collector and the base bias circuits are energized by a potential source which is maintained at a predetermined fixed voltage independent of the variations in amplifier load, line voltage and ambient temperature. The negative end of zener diode Z is connected by resistor R7x to the negative terminal of an auxiliary power supply B2x having its positive terminal connected to the ground bus G.

The second pre-drive stage comprises an NPN transistor Q2x also operating in the Class A common-emitter mode. The base of transistor Q2x is direct-coupled by lead 201 to the collector of the first pre-drive transistor Qlx. The emitter of transistor Q2x is connected through a bias resistor Rltlx to the negative terminal of the negative section Blx of the split main power supply indicated schematically by the battery symbols. The collector of transistor Q2x is connected to the upper end of a temperature compensating bias diode D having its lower end connected to the series-connected load resistors R8x, Rx. A conventional bootstrapping capacitor C3x has its lower end connected to the junction of collector load resistors R8x, R9x and its upper end is connected through lead 215 to the output bus 0.

The circuit of FIG. 2 comprises two drive stages, the first drive stage being the complementary symmetry type and including an NPN drive transistor Q3x and a PNP drive transistor Q ix. Bias resistors Rllfix, R17x extend from the respective emitters of transistors Q3x, Q4x to output bus 0. The collector of NPN drive transistor Q3x is connected by resistor RlSx to the positive terminal of the positive section Blx of the main power supply.

The second drive stage comprises four PNP transistors QSx, Q6x, Q7x, Q8x. The base of the lowermost drive transistor Q5x is direct-coupled by lead 204 to the collector of transistor Q3x and the base of drive transistor Qix is direct-coupled by lead 205 to the emitter of transistor Q4x. Resistor R18x extends from the emitter of drive transistor Q5x to the positive terminal of power supply section Blx and resistor Rl9x extends from the emitter of drive transistor Q'7x to the output bus 0. The collector of transistor ()6): is connected to the output bus 0 and the collector of transistor Q8x is connected to the negative terminal of the negative section Blx of the main power supply.

The potential of the base of drive transistor Q6x is maintained midway between the potentials of ground bus 0 and the positive terminal of the power supply by a feedback network energized by the output of the amplifier and comprising the voltage divider resistors Rllx, R12x arranged in series from the power supply positive terminal to the output bus 0. The base of transistor Q6x is connected by lead 213 to the junction of voltage divider resistors Rllx, R12x. The resistance values of the latter are preferably approximately equal.

In a similar manner the potential of the base of drive transistor Q8x is maintained approximately midway between the potentials of output bus 0 and the power supply negative terminal by means of a feedback network comprising the voltage divider resistors R13x, R14x extending from output bus 0 to the power supply negative terminal. The base of drive transistor QSx is connected by lead 214 to the junction of resistors R13x, R14x. The resistance values of the latter are also preferably approximately equal. As a result, the transistors of both the second drive stage and the output stage will share approximately equally both the quiescent D.C. potentials and the varying A.C. potentials as the output bus 0 swings in response to a signal input to the amplifier.

The output stage of the circuit of FIG. 2 comprises four power transistors Q9x, Qltlx, Qllx, Q12): arranged in series. The emitter of the lowermost output transistor Q9x is connected through bias resistor R20x and fuse Flx to the power supply positive terminal and its collector is connected to the emitter of the next higher output transistor Qltlx. The collector of the latter is connected to the output terminal 01x to which the emitter of output transistor Qllx is also connected through the bias resistor RZlx and fuse F2x. The collector of output transistor Qllx is connected to the emitter of the uppermost output transistor Q12x, the latter having its collector connected to the power supply negative terminal.

The second drive stage is direct-coupled to the output stage in the following manner so as to form four Darlington compound pairs. The emitter of each of the second drive stage transistors QSx, Q6x, Q7x, Q8x is direct-cow pled by one of the leads 207, 209, 210, 212 to the base of a respective one of the output transistors Q91, Q10x, Qllx, Q12x. The collector of the first drive stage transistor Q4x is connected by lead 206 to the collector of the second drive stage transistor Q7x which is in turn connected by lead 211 to the collector of output transistor Qllx. The collector of drive transistor Q52: is connected by lead 208 to the collector of output transistor Q9x.

Output terminal 01x is directly connected to the midpoint of the output stage at the collector of transistor Qltlx. The other output terminal 02x is grounded as shown. Hence the loudspeaker S is D.C.-coupled to the amplifier so as to provide all of the advantages inherent in this arrangement and discussed above with respect to the circuit of FIG. 1. In order to maintain the output terminals 01x, 02x at the same DC. potential so as to prevent the flow of DC. current in the voice-coil of the loudspeaker, there are provided two feedback systems.

The first feedback system arises from the deegeneration inherent in the resistors R3x, R4x in the emitter circuit of transistor Qlx. These resistors counteract any tendency for the collector current of transistor Qlx to vary with changes in temperature. The zener diode Z maintains the upper end of collector load resistor R52: and the upper end of potentiometer Px at a substantially fixed voltage. Therefore the potential of collector Qlx is maintained substantially constant and hence the DC. potential applied to the base of the second pre-drive transistor Q2x remains substantially fixed.

The second feedback system for maintaining output terminals 01x, 02x at the same DC potential comprises a DC. feedback network extending from the output 01x to the emitter of the first transistor Qlx. This network includes a feedback resistor Rfix, connected in parallel with the usual phase-advance capacitor C2x, and having one end connected to output 01x and its opposite end connected to the emitter of transistor Qlx. This feedback network is degenerative so as to counteract any tendency of DC. voltage of output 01x to vary from ground potential. This feedback network also serves the additional function of providing A.C. negative feedback for the usual purposes of reducing distortion, improving frequency response, and reducing the output impedance of the amplifier. In order to reduce the A.C. feedback to an amount which will provide an adequate stability margin the lower emitter resistor R32: is bypassed by the capacitor Clx.

Referring now to FIGS. 3 and 4, there are disclosed two embodiments of a double-loop feedback arrangement made more feasible by the improved phase shift characterist-ics due to the D.C.-coupling feature of the amplifier circuits shown in FIGS. 1 and 2. Each of these feedback systems comprises an inner negative feedback network extending around the final several stages and providing, in addition to the usual feedback advantages of reduced distortion, an improved pole-zero configuration for this portion of the amplifier circuit so as to permit the application of still more feedback through the outer negative feedback network. That is, the inner feedback loop provides an added stability margin for the outer feedback loop.

Referring now to FIG. 3 in more detail, the reference character A2 designates a symbol for an amplifier which may be in accordance with the circuit of FIG. 1. The circuit of FIG. 2 may also be utilized provided that one of the preamplifier stages be omitted to take into account the extra phase inversion provided by the extra common-emitter pre-drive stage in this amplifier embodiment, as will be explained below.

The double-feedback system of FIG. 3 further comprises a first preamplifier stage operating in the Class A common-emitter mode and comprising a transistor Q17 having its base connected by resistor R45 to the input terminal I7. Resistor R45 prevents the feedback current from fiowing into the preceding stage (not shown) instead of into the base of Q17. The other input terminal I8 is connected to ground bus G. Also connected to the latter are a base bias resistor R46 and an emitter resistor R48. The latter is bypassed by a capacitor C19. The other base bias resistor R47 extends to a potential source B Extending from the latter to the collector of transistor Q17 is a load resistor R49.

The collector of transistor Q17 is direct-coupled to the base of a transistor Q18 constituting a second preamplifier stage and having a collector load resistor R51 connected to the potential source 13 and an emitter resistor R50 connected to ground bus G. The collector of transistor Q18 is coupled by capacitor C20 to the input terminal Ile of amplifier A2.

Said amplifier input terminal He may correspond to terminal I1 of FIG. 1. Amplifier A2 is shown grounded to bus G by lead 123. The output terminal Ole of amplifier A2 is direct-coupled to speaker terminal T1 of loudspeaker S by lead 128. Output terminal Ole may correspond to output terminal 01 of FIG. 1. The other speaker terminal T2 is grounded to bus G.

The inner feedback network comprises a feedback resistor R52 in parallel with a capacitor C21 and connected by lead 124 to the output terminal 01c and by lead 125 to the emitter of preamplifier transistor Q18. The outer feedback network comprises a feedback resistor R53 in parallel with a capacitor C22 and connected by lead 126 to output terminal Ole and by lead 127 to the base of the first preamplifier transistor Q17.

The transistor and network parameters may be selected so as to provide either of two general modes of operation. Either the closed-loop transfer function of that portion of the circuit enclosed by the inner feedback network, hereinafter referred to as the inner transfer function, has a substantially higher cutoff frequency than that of a preceding preamplifier stage, or said inner transfer function has a substantially lower cutoff frequency than all of the preceding preamplifier stages.

In the first mode of operation, capacitor C21 provides a phase-advance to improve the pole-zero configuration of the inner transfer function. The inner feedback network raises the high-frequency cutoff of this portion of the circuit by approximately the amount of the inner feedback. Transistor Q17 should be of a type having a beta cutoff frequency substantially lower than the cutoff frequency to which the inner transfer function is boosted by the inner feedback network. Alternatively, this rolloif in the first preamplifier stage may be provided by a filter network such as a resistor and a capacitor connected in series between the collector of Q17 and ground.

Therefore, as the signal frequency is increased, attenuation will occur initially only in the first preamplifier stage comprising transistor Q17, and the remaining stages enclosed by the inner feedback network will have neither frequency response attenuation nor phase shift to any significant degree until the signal frequency is increased to about a decade beyond the beta cutoff frequency of transistor Q17. By this time, transistor Q17 has provided sufficient attenuation so that the outer loop gain is reduced to below unity before the phase shift of the inner transfer function reaches degrees. Since the maximum phase shift in the first preamplifier stage is 90 degrees, the overall loop gain will be reduced to below unity before the outer loop phase shift reaches 180 degrees, and hence stability for the outer feedback loop is assured by the increased high frequency response of the inner transfer function provided by the inner feedback network.

The alternative mode of operation requires that the upper cutoff frequencies of all preceding preamplifier stages be substantially greater than that of the inner transfer function. In the circuit of FIG. 3 this condition is satisfied by providing an early frequency rolloff in at least one of the stages of amplifier A2 and by selecting for Q17 a transistor having a very high beta cutoff frequency. In

this event the inner transfer function provides sufiiclent attenuation so that the outer loop gain is reduced to below unity before the phase shift of Q17 reaches 90 degrees, and hence before the outer loop phase shift reaches 180 degrees.

Still another form of double-loop feedback system is shown in FIG. 4 wherein the second preamplifier stage is direct-coupled to the amplifier and the first preamplifier stage is capacitor-coupled to the second preamplifier stage, so as to be the reverse of the coupling arrangement of FIG. 3. More specifically, a first preamplifier transistor Q22 is provided with 'base bias resistors R70, R80, an emitter resistor R81 bypassed by a capacitor C32, and a collector resistor R82 connected to a potential source 13 The base of Q22 is connected to input terminal I13 by resistor R69 and the other input terminal I14 is grounded to bus G.

tor C33 to the base of a second preamplifier transistor Q23 having an emitter resistor R84 and a base bias resistor R83 connected to ground bus G and a collector load resistor R85 and a base bias resistor in the formof potentiometer Ph connected to a potential source The latter is preferably the junction of the positive end of zener diode Z1 of FIG. 1 and resistor R6 in the event that the amplifier A5 of FIG. 4 is in the form'of said FIG. 1.

This circuit will be modified so as to eliminate therefrom the base bias network of the pre-drive stage including components D1, R1, R2 and P of FIG. 1. This bias network is omitted because the collector of transistor Q23 is direct-coupled to the base of the pre-drive stage transistor so as to set the bias of said base. The DC. poten-. tial of output terminal 01h of amplifier A5 is set to ground level by adjustment of potentiometer Ph. Amplifier A5 is grounded to bus G by lead 139. Terminal T1 of loudspeaker S is direct-coupled to output terminal 0111 by lead 144 and the other speaker terminal T2 is grounded.

The inner feedback network comprises a resistor R86 in parallel with a capacitor C34 and having one end connected by lead to output terminal 01h, and its other end connected by lead 141 to the emitter of transistor Q23. Because of the D.C.-coupling between transistor Q23 and amplifier A5, the inner feedback network of FIG. 4 serves two functions. In addition to the function of the inner feedback network of FIG. 3 as described above in connection with the latter, the inner feedback network of FIG. 4 also provides D.C. negative feedback which helps to maintain the DC. potential of output terminal 01h at ground level so as to prevent direct current in the loudspeaker voice-coil. The outer feedback network is similar to that of FIG. 3 and comprises a resistor R87 in parallel with a phase-advance capacitor C35 and connected at one end to output terminal 01h by lead 142 and at the other to the base of transistor Q22 by lead 143.

Referring now to FIGS. 5 to 7, there is shown the manner in which two amplifiers in accordance with the prescut invention may be connected alternatively in either a stereo mode (FIG. to provide two independent channels, or in a parallel mode (FIG. 6) to drive a single lowimpedance speaker, or in a series mode (FIG. 7) to drive a single high-impedance speaker. This alternative connection capability is made feasible by the fact that the present invention maintains the D.C. potential of the amplifier output terminals at a constant ground level.

Describing in more detail first the stereo mode connection of FIG. 5, amplifiers A6 and A7 may be identical and may embody any of the amplifier circuits described above. Each amplifier is preceded by a preamplifier stage comprising a transistor Q24 and Q25, respectively. Each of the latter is provided with an emitter resistor R88 and R90 and a collector resistor R89 and R91, respectively. Resistor R91 is connected to a potential source Bgand resistor R89 is connected to the latter by lead 145. The 'base of Q24 is coupled by capacitor C36 to the input terminal I and the base of Q25 is similarly coupled by capacitor C37 to the input terminal I17. The two input terminals 116 and I18 are grounded to bus G. The emitter of Q24 is coupled by capacitor C38 to the input of amplifier A6 and the emitter of Q25 is coupled by capacitor C39 to the input of amplifier A7. Amplifier A6 is grounded to bus G by lead 148 and amplifier A7 is similarly grounded by lead 149.

The reference letters S and S indicate respectively the loudspeakers for the left and right channels. The terminal T1 of loudspeaker S is direct-coupled by lead 146 to the output of amplifier A6 and the terminal T 1a of loudspeaker S is direct-coupled by lead 147 to the output of amplifier A7. The other speaker terminals T2 and T2a are grounded to bus G. It will thus be seen that in the connection arrangement of FIG. 5 transistors Q24 and Q25 operate in the emitter-follower mode and that there are two independent channels each driving a respective one of the loudspeakers S, S.

Describing now the parallel connection of FIG. 6, the same components as in FIG. 5 are employed but are connected in a different mode by a switching arrangement which is not shown since it would be obvious to one skilled in the art in view of the disclosed circuit diagram. Input terminal I19 is coupled both to the base of Q24 by capacitor C36 and to the base of Q25 by lead 152 and capacitor C37. The other input terminal I is grounded.

The terminal T1 of a single loudspeaker S is directcoupled both to the output of amplifier A6 by lead 150 and also to the output of amplifier A7 by lead 151. The other speaker terminal T2 is grounded. It will thus be seen that the amplifiers A6, A7 are driven in phase from a single pair of input terminals and are D.C.-coupled in parallel to each other and to the loudspeaker S.

Describing now the series connection of FIG. 7, the latter differs from that of FIG. 6 in only two respects. First, the loudspeaker S has one terminal T1 directcoupled by lead 153 to the output of amplifier A6 and its other terminal T2 direct-coupled by lead 154 to the output of amplifier A7. Second, the input of amplifier is coupled by capacitor C39 to the collector of transistor Q25, instead of to the emitter of the latter as in FIG. 6. The second change provides a phase reversal in transistor Q so that the amplifiers A6 and A7 are driven in phase opposition to form a push-pull full-bridge output circuit for driving loudspeaker S.

Since amplifiers A6, A7 are connected in parallel in the connection of FIG. 6, each amplifier will see a load impedance of twice the impedance of loudspeaker S. Hence if the latter is of relatively low impedance, such as, for example, four ohms, each amplifier will have an effective load of eight ohms which is a better match for most power transistors. However, in the connection of FIG. 7 amplifiers A6, A7 are connected in series across the load, and hence each amplifier will have an effective load impedance of one-half the impedance of loudspeaker S. Therefore if the latter is of the high impedance type,

'14 such as sixteen ohms, each amplifier will have the better matched effective load of eight ohms.

It will thus be seen that the consumer has his choice of either two low-power stereo channels, or a single highpower monophonic amplifier with an impedance-matching capability for increased power output. This modular arrangement enables the consumer to start his stereo system at a minimum expense by purchasing a pair of channels and utilizing the stereo mode connection, and then to increase the power capability of the system by purchasing another pair of channels and utilizing each pair in either the parallel or series mode as may be suitable for the speaker impedances. In this way the system is improved without sacrificing the equipment originally purchased from considerations of modest initial cost.

Referring now to FIG. 8, the circuit embodiment there disclosed comprises an emitter-follower input stage including an NPN transistor Qla having its base coupled through capacitor Clrz to the hot input terminal 11a. The other input terminal 12a is grounded as shown. The emitter of transistor Qla is provided with a load resistor R3a extending to ground.

There is provided a fixed regulated potential source at the negative electrode of a zener diode Za having its positive electrode connected to ground. A resistor R4a extends from the negative terminal B of a conventional nonregulated power supply (not shown) to the negative electrode of zener diode Zn. In order to filter out any residual ripple, a capacitor C3a may be connected in parallel across zener diode Za. The base of transistor Qla is biased by connection to the junction of a pair of resistors Rla, R2a connected in series between the negative electrode of zener diode Za and ground. The collector of transistor Qla may also be connected to said negative electrode of zener diode Zn.

The second stage including PNP transistor Q2a is the first of the two common-emitter stages. The base of transistor Q2a is coupled by capacitor C2a to the emitter of transistor Qla and is biased by connection to the junction of the series-connected pair of resistors R5a and Pa, the latter preferably being variable in the form of a potentiometer for adjustment of the quiescent direct-current level of the output stage in a manner to be described. The negative end of potentiometer Pa is connected to the junction of resistor R4a and zener diode Za, and the positive end of resistor R5a is connected to ground. A collector load resistor R9a extends from the collector of transistor Q2a to the negative supply terminal B The second common-emitter stage comprises an NPN transistor Q3a having its base direct-current coupled to the collector of the first common-emitter transistor Q2a. The emitter of transistor Q30 is connected through bias resistor R12a to AC. ground at the negative supply terminal B and a conventional bypass capacitor C7a is connected in parallel across resistor R12a. The collector load impedance of transistor Q3a comprises the series connection of resistors R10a, Rlla and temperature-compensating bias diodes Dla, D2a. The upper end of resistor Rltla is connected to the positive power supply terminal B+ and the lower end of diode D2a is connected to the collector of transistor Q3a. If warranted by the phaseshift characteristics of the amplifier, a capacitor C5a may be connected between the collector and base of transistor Q3a to improve the feedback stability margin at high frequencies. A conventional bootstrapping capacitor C6a extends from output bus 0a to the junction of resistors R1011, Rlla.

Connected in series between the emitter of transistor Q2a and the output bus 0a are a pair of resistors R7a, RSa. The junction of the latter is grounded with respect to alternating-current signals by a capacitor C4a. A resistor R6a is connected at one end to the output bus 0a and at the other end to the emitter of transistor Q2a. Resistors R7a, R8a thus provide direct-current feedback from the output to the emitter of transistor Q2a to main- 15 tain the quiescent direct-current potential of output terminal 0111 at ground level and thereby obviate any substantial direct current through the voice-coil of speaker S. At signal frequencies alternating current feedback is provided by resistors R6a, R7a.

Transistors Q2a and Q3a are of opposite polarity types, the former being shown for purposes of illustration as of type PNP whereas the latter is shown as type NPN. This enables the base of transistor Q3a to be directcurrent-coupled to the collector of transistor Q2a while the emitter of the latter may be direct-current-coupled through the feedback network R'la, R8a to the output 01a of the amplifier.

The drive stage comprises a pair of complementary transistors Q4a and QSa, the former being shown as type NPN and the latter as type PNP. The collector of transistor Q4a is connected to the positive power supply terminal B+ and its emitter is connected through bias resistor R13a to output bus a. The emitter of: transistor Q5a is connected to output bus Oa by resistor R14a and its collector is connected by bias resistor RlSu to the negative terminal B The base of transistor Q4a is direct-current-coupled to the junction of resistor Rlla and diode Dla, and the base of transistor Q50 is direct-currentcoupled to the collector of transistor Q3a. Drive transistors Q4zt, QSa operate in a push-pull mode and are preferably biased for Class AB operation in the usual manner.

The output stage comprises a pair of output transistors Qfia and Q9a connected in series between the positive power supply terminal B+ and the negative terminal 3 in the conventional single-ended push-pull configuration. The collector of transistor QSa is connected to the positive supply terminal B+ and its emitter is connected through resistor Ra to the output bus Oa. Also connected to the latter is the collector of transistor Q9a having its emitter connected through resistor RZla to the negative supply terminal B The base of transistor Q8a is direct-current-coupled to the emitter of NPN drive transistor Q4a and the base of output transistor Q9a is similarly direct-current-coupled to the collector of PNP drive transistor Q50. The resulting combination of drive stage Q4a, QSa and output stage (1811, Q9a constitutes the well-known quasi-complementary-symmetry arrangement.

A load, such as illustrated by loudspeaker S, is directcoupled between the hot output terminal Ola and the grounded output terminal 02a; It will thus be seen that if the quiescent direct-current potential of output terminal 01a departs substantially from ground level a direct current will flow through the voice-coil of speaker S. This current will bias the cone of speaker S to an off-center position so as to increase the harmonic and intermodulation distortion characteristics of the speaker. Therefore it is important that the quiescent direct-current potential of output terminal Ola be maintained substantially at ground level.

This is achieved by a large amount of direct-current feedback from the output terminal Ola through the feedback network R712, RSa to the emitter of transistor Q2a which is made possible by the existence of two stages of gain within the feedback loop; that is, the overall forward transmission gain is equal to the product of the individual voltage gains of the two common-emitter stages Q2a and OM. This large amount of direct-current feedback does not affect the stability characteristics at signal frequencies because at such frequencies the directcurrent feedback is bypassed to ground by capacitor C411 and the amount of alternating-current feedback may be selected as desired by choosing the proper magnitude ratio of resistor R6a to resistor R7a.

Due to economic considerations, the respective potentials at power supply terminals B-land B are unregulated, so as to undergo a substantial voltage drop and to contain a highripple content when substantial power is drawn from the supply. Injection of the power supply ripple into the first two stages Qla, Q2a is pret vented by the bias network including Zener diode Za, thereby avoiding the amplification of the ripple and maintaining a low hum level at the output of the amplifier.

to regulation under load or voltage variations in the supply mains. This is important because the direct-current feedback network is arranged so that the polential at output terminal 01a is compared with that at thebase of transistor Q2a which is therefore the reference potential. The feedback network corrects for any deviation of the potential of output terminal 01a from this reference potential and if the latter is substantially fixed the potential of output terminal Ola may be maintained at the proper ground level so as to prevent quiescent direct current through the voice-coil of. speaker S.

Another advantage of the circuit of FIG. 8 is that the distortion may be reduced to extremely small levels by the application of a large amount of alternating-current feedback without the usual problems of oscillation and poor transient response due to an inadequate stability margin. With respect to low frequencies, unstable operation such as motor boating, blocking bias changes,

and other symptoms of instability cannot occur because t the circuit has no low-frequency phaseshift-producingu coupling capacitors within the feedback loop.

With respect to high frequencies, the circuit arrangement provides stability by causing the second commonemitter transistor (23:1 to roll off at a much lower frequency than the other stages so that the overall amplifer gain is reduced to unity before the total phase shift reaches This is achieved by driving transistor Q3a from a relatively high-impedance source provided by the preceding first common-emitter stage Q2a. The impedance of this driving source is substantially the value of resistor R941 which is preferably about several thousand ohms. Hence the second common-emitter stage Q3a will roll off substantially at the beta cut-off frequency. On the other hand, the first common-emitter stage Q2a is driven by the relatively low-impedance source provided by the preceding emitter-follower transistor Qla. Hence transistor Q2a has an effective high-frequency cut-off substantially beyond the beta cut-off frequency. The drive-output stage configurationcomprising transistors Q4a, QSa and Q8a, Q9a also operates in the emitter follower mode, so that the voltage response of this1cort figuration may also extend far beyond the beta cut-off frequency of the second-common-emitter stage (2311. As

a result, alternating-current feedback resistors R6a, R7a

may be selected so as to apply an unusually large amount of feedback without causing oscillation, transient ringing, or other symptoms of insufficient stability margin at the high-frequency end of the spectrum.

It will be seen that when drive transistor Q4a is cut ofi, no current flows through bias resistor R13a and hence the base of output transistor Q8a is at the same potential as output bus Oa. Hence in the absence of the circuitry to be described below the base and emitter of transistor Q8a would be substantially at the same potential and no reverse bias would be applied to the base-emitter junction of output transistor Q8a during its off half of the cycle. Similarly, when drive transistor QSa is cut-off, no current flows through bias resistor R15a and hence the base and emitter of output transistor Q9a would be substantially at the same potential so that no reverse bias would be applied to the baseemitter junction of transistor Q9a during its off portion of the cycle.

As a result, at high frequencies excess minority car-- riers would remain stored in the respective bases of output transistors Q8a, Q9a during their off portions of the cycle in the absence of any reverse bias to drain 17 off the trapped majority carriers which keep an equal number of minority carriers in the base in accordance with the requirement for space-charge neutrality. A substantial collector current would thereby flow through each output transistor Q8a, Q9a when it should be cutolf, at which time the transistor is subjected to a relatively high collector voltage. This would cause excessive heat dissipation and destruction of output transistors Q8a, Q9a when subjected to a continuous high-level signal at a high audio frequency.

To obviate this problem, the circuit of FIG. 8 is provided with a pair of transistors Q8a and Q7a which function to provide a reverse bias at the respective baseemitter junctions of output transistors Q8a, Q9a during their respective off portions of the push-pull cycle. The collector of transistor Q6a is connected to the base of output transistor QSrz and the base of transistor Q6a is connected to the collector of driver transistor Q50. The emitter of transistor Q60 is connected through bias resistor R16a to the negative supply terminal B The collector of transistor Q7a is connected to the base of transistor Q9a. The base of transistor Q7a is connected through resistor R17a to the output bus a and also through resistor R1811 to an auxiliary negative supply terminal B AlsO connected to the latter through a bias resistor R19a is the emitter of transistor Q'7a.

During the off phase of output transistor Q8a, a substantial current is drawn through bias resistor R150 by drive transistor QSa so as to raise the potential at the base of transistor Q6a thereby turning the latter on to draw current from output bus 0a through resistor R13a to the collector of transistor Qfia. As a result the potential at the base of output transistor Q8a will be below that of output bus 0a by the amount of the potential drop through resistor R1311, thereby reversebiasing the base-emitter junction of output transistor Q8a. Similarly, during the otf phase of output transistor Q9a the potential of output bus Oa rises so as to raise the potential at the base of transistor Q7a thereby turning the latter on so as to draw current from the negative supply terminal B through the bias resistor RlSa to the collector of transistor Q7a thereby reverse-biasing the base-emitter junction of transistor Q9a by approximately the amount of the voltage drop in resistor R1541.

Referring now to FIG. 9, there is shown a modified form of the invention wherein the base of the first common-emitter stage is biased by direct coupling to the emitter of a preceding emitter-follower stage. The input terminal 11b is coupled by a capacitor Clb and a re sistor Rlb to the base of an NPN transistor Qlb operating in the emitter-follower mode. The base of transistor 01b is biased by a variable resistance in the form of a potentiometer Pb connected at one end to ground and at the other end in series with a resistor R2b extending to said base. The other input terminal 12b is grounded as shown. The collector of transistor Qlb is connected to ground and its emitter is connected through emitter load resistor R3b and filter resistor R1712 to the negative supply terminal B- of a conventional non-regulated power supply (not shown). A capacitor CSb extends from the junction of resistors R311, R171) to ground to bypass the ripple at supply terminal B.

The emitter of transistor Qlb is direct-coupled to the base of a PNP transistor Q2b constituting the first common-e-mitter stage. The emitter of transistor Q2b is connected through the series combination of resistor R4b and capacitor C2b to ground, and its collector is connected to the filtered negative supply through the load resistor RSb. The collector of transistor Q2b is directcoupled to the base of an NPN transistor Q3b constituting the second common-emitter stage and having its emitter connected to the filtered negative supply through bias resistor R9b in parallel with bypass capacitor C41). The collector load impedance of transistor Q3b comprises a series network including load resistors R7b, R8b and ing potentiometer Pb.

18 temperature-compensating diodes Dlb, D2b extending from the positive supply terminal B+ to said collector. A conventional bootstrapping capacitor C3b has one end connected to the output bus Ob extending from the hot output terminal 01b and its other end connected to the,

junction of resistors R7b, R8b.

It will be noted that each transistor of the cascade? arrangement Qlb, Q2b, Q3b is of complementary polarity type with respect to the preceding transistor. For purposes of illustration, transistors Qlb and Q3b are shown to be of NPN type whereas transistor Q2b is illustrated as of PNP type. However, it will be understood that these polarity types may be reversed if desired. As a result of this arrangement direct-coupling between these stages with maintenance of proper bias conditions can be obtained without the use of Zener diodes, regulated power supplies and similar expedients.

The drive stage comprises a pair of complementary transistors Q4!) and Q51), for purposes of illustration the former being of NPN type and the latter of PNP type. The collector of transistor Q4b is connected to the positive supply terminal B+ and its emitter is connected through bias resistor R10b to output bus Ob. Also connected to the latter through resistor Rllb is the emitter of transistor 05b having its collector connected through bias resistor R12b to the negative supply terminal B. The base of drive transistor Q4b is direct-coupled to the junction of resistor Q8b and diode Dlb, and the base of drive transistor QSb is similarly direct-coupled to the junction of diode D2!) and the collector of transistor Q31).

The next stage has a novel function and will be referred to by the coined term quasi-output stage. It comprises a pair of transistors Q6b, Q7b of the same polarity type. The collector of transistor Q6b is connected to the positive supply terminal B+ and its emitter is connected to output bus Ob through resistor R13b. Also connected to output bus Ob is the collector of transistor Q7b having its emitter connected through resistor R141) to the negative supply terminal B-. The base of quasi-output transistor Q6b is direct-coupled to the emitter of drive transistor Q4b and the base of the other quasioutput transistor Q7b is similarly direct-coupled to the collector of drive transistor QSb. j

The output stage comprises a pair of transistors Q8b, Q9b of the same polarity type. The collector of transistor Q8b is connected to the positive supply terminal B+ and its emitter is connected through resistor R15b to output bus Ob. Also connected to the latter is the collector of output transistor Q9b having its emitter connetced through bias resistor R161? to the negative supply terminal B. The base of output transistor Q8b is directcoupled to the emitter of quasi-output transistor Q6b and the base of output transistor Q9b is direct-coupled to the emitter of quasi-output transistor. Q7b.

The reference letter S indicates a loudspeaker or other load having one end connected to the hot output terminal 01b and its other end connected to the second output terminal 02b which may be grounded as shown. In order to prevent the flow of quiescent direct current through the voice-coil of speaker S, the quiescent potential of output terminal Olb must be maintained substantially at ground level. The bias conditions of the various stages may be initially set so as to adjust the potential output terminal 01b to ground level by vary- Since the base of transistor Qlb is connected to ground through resistor R2!) and potentiometer Pb, the base of transistor Qlb is maintained at a substantially fixed potential with respect to ground. Because transistor Qlb operates in the emitter-follower mode, its emitter will be maintained at a substantially constant predetermined potential difference from that of its base independently of any potential variation and ripple component at the negative power supply terminal B. Asa result the base of the first common-emitter transistor Q2b is main-.

tained at a relatively fixed reference potential.

The potential at the emitter of transistor Q2b is constantly compared with this fixed reference potential at its base by means of the direct-current feedback network comprising feedback resistor R6b extending from output bus Ob to the emitter of transistor Q2b. For example, if the potential of output bus Ob tends to rise above ground level the resulting increased voltage across the base-emitter junction of transistor Q2b causes more collector current to flow therethrough thereby increasing the voltage drop across resistor RSb and increasing the potential at the collector of transistor Q2b. This in turn raises the potential at the .base of transistor Q3b so as to increase the collector current therethrough and thereby increase the voltage drop across load resistors R7b, RSb. This lowers the potential at the collector of transistor Q3b. Since the remaining drive, quasi-output and output stages operate effectively in the emitter-follower mode, the lower potential of the collector of transistor Q3b in turn results in a lower potential of output has Ob so as to counteract the originally assumed tendency of the potential of output bus Ob to rise. It will be obvious that the feedback arrangement has the opposite effect in the event that there is a tendency for the potential of output bus Ob to fall. The unity feedback provided by the resistor R6b is reduced at signal frequencies by capacitor C2b and resistor R417. The relative magnitudes of resistors R4b, R6b are selected so as to attain the desired amount of alternating-current feed-back.

The amount of this alternating-current feedback may be made extremely large so as to reduce the distortion of the amplifier to almost unmeasurable levels while retaining an ample stability margin which will obviate any tendency to oscillation, ringing, or transient distortion, as discussed above with respect to the circuit modification of FIG. 8. At the high frequency end the circuit of FIG. 9 provides an additional stability margin by virtue of the fact that the lower output transistor Q9b is driven by a source of much lower impedance than that which drives the lower output transistor (19:: in FIG. 8. That is, the magnitude of resistor R15a of FIG. 8 is preferably about one hundred ohms which is the source of impedance seen at the base of output transistor Q9a. This causes output transistor Q9a to roll off at substantially its beta cutoff frequency. In the present state of the art, this beta cutoff frequency for output transistors of reasonable cost is approximately one octave below the upper audio limit, generally regard as 20 kHz. The resulting phase shift in the circuit of FIG. 8 substantially reduces the stability margin at high frequencies.

In the circuit of FIG. 9, on the other hand, the lower output transistor Q9b is driven by a relatively low source impedance of only a few ohms. As will be explained below, the magnitude of resistor -Rl4b is preferably in the range of about 2 to ohms. Furthermore, the base of transistor Q9b is driven by the emitter of quasi-output transistor Q7b operating in the emitter-follower mode which provides 'a low source of impedance. As a result of the low impedance of the driving source seen by the base of output transistor Q9b the effective rolloff frequency of the latter extends far beyond the beta cutoff frequency of the transistor and the resulting reduction of phase shift improves the stability margin of the circuit of FIG. 9 as compared to that of FIG. 8.

In FIG. 9 the quasi-output stage Q61), Q7b and output stage QSb, Q9b function in the novel manner disclosed in my prior copending application entitled, Transistor Audio Amplifier with Power-Division Output Stages, Ser. No. 501,515, filed Oct. 22, 1965. The quasioutput stage is normally biased with a small quiescent current for Class AB push pull operation so as to supply power to the load S at low signal levels while at these levels the output stage Q8b, Q9b is entirely cut off. The load current flows alternately through resistors R131),

R14b which are preferably in the range of about 2 to 10 ohms eaoh. At higher signal levels the respective voltages across R13b, R14b attain a magnitude sufficient to turn on respective output transistors Q8b, Q9b so that the latter then become active and supply power to the load S at higher signal levels, at which time quasioutput transistors Q6b, Q7b function as drive transistors for the output stage Q8b, Q9b.

Since there is. substantially no quiescent bias current in the output stage QSb, Q9b even at higher operating junction temperatures, the problems of thermal runaway, excessive power dissipation and reduced power ratings are avoided. Because the low-level power is provided by the quasi-output stage Qob, Q7b which is biased with 'a sufficiently large quiescent currentfor this purpose, crossover distortion at low levels is minimized. No problem of excessive bias current can arise in quasi-output stage Qfib, Q7b because this stage provides only small amounts of power and therefore operates at relatively cool jnnc tion temperatures so that the transfer characteristic of the transistors does not vary significantly. For further details as to the mode of operation and advantages of this drive output arrangement reference is made to said prior copending application Ser. No. 501,515.

Alternatively, transistors Q6b, Q7b of FIG. 9 may function as a conventional emitter-follower drive stage and the output stage Q8b, Q9b may be provided with a small quiescent bias current so as to operate in the Class 3 AB mode in the usual manner. In this event emitter resistors R13b, R14b are of a magnitude suflicient to bias output transistors Q8b, Q9b at this quiescent pointya typical magnitude for these resistors being about two hundred ohms. In this arrangement the lower output transistor Q9b will still be driven by a relatively low source impedance since transistor Q7b operates in an emitterfollower mode having a low output impedance. As a result the extended high-frequency response and reduced phase shift of transistor Q9b discussed above are .retained.

In all of theembodiments disclosed in FIGS. 1 to 9 described above and in FIGS. 1a, 2a, 3a and 4a to be described below the critical importance of the drive stage for the practical utilization of the subject circuits in most applications, and the coaction of the drive stage with the directcurrent coupling arrangement for this purpose, will not be apparent to those skilled in the art and are now described. Although providing obvious secondary advantages such as increased current gain and, in some embodiments, phase inversion, the primary need for the drive stage is to protect the loudspeaker from being damaged in the event of failure of an output transistor. This requirement arises because of the lack of an output coupling capacitor or output transformer to isolate the loudspeaker from the power supply in the event of a shorted output transistor. The drive stage is able to provide the required protection of the loudspeaker because of the direct'current transmission path through the amplifier. Furthermore, the protective effect is greatly enhanced by the direct-current feedback network, as explained in detail below.

The importance of this safety feature cannot be underestimated. The expense and inconvenience of delivering a twenty-pound amplifier to a repair shop for replacement of a few transistors is far less than that involved in returning a one-hundred-pound speaker system to the factory for replacement of the voice-coil, cone and suspension. Furthermore, although the consumer has learned to accept the risk of transistor amplifier failure as inherent in the price he must pay for its advantages over tube amplifiers, repntablemanufacturers are understanda'bly reluctant to market an amplifier which might also destroy the loudspeaker..

The operation of the subject protective arrangement will be described with respect to FIG. 1, it being understood that the same protective operation is provided by 21 the other embodiments of FIGS. 2 to 4, 8 and 9 and that a substantially similar operation is provided by the embodiments of FIGS. 1a, 2a, 3a and 4a to be described below. Referring to FIG. 1, let it be assumed that output transistor Q5 undergoes a short-circuit failure due to second breakdown or other cause. In this event output terminal 01 will be connected practically directly to the negative terminal of the power supply B1 since the magnitude of emitter resistor R12 is generally only about onehalf ohm. Almost the entire voltage of power supply Bl is thereby impressed across the voice-coil of loudspeaker S so that in the absence of some protective circuit arrangement a large surge of current will flow through the voice-coil so as to burn out the voice-coil or to cause mechanical damage to the voice-coil or the cone suspension when the cone bottoms. The protective arrangement of the present invention prevents a large current surge through the voice-coil, in the following manner.

Since the bases of drive stage transistors Q2, Q3 are direct-current coupled by leads 11, 12 to the collector circuit of transistor Q1 said bases are maintained at the respective potentials determined by the signal applied to the input terminal I1 of the amplifier notwithstanding the short-circuit failure of output transistor Q5. The tendency of said bases to resist potential deviation due to output transistor failure is further enhanced by the direct-current feedback network comprising lead 16, potentiometer Pb and resistor R2 so as to apply to the base of transistor Q1 a feedback signal counteracting any tendency of the potentials of the drive stage bases to go negative. Therefore as the potential of output terminal 01 commences to go negative in response to the short-circuit failure of output transistor Q5 the emitter of drive transistor Q2 goes negative therewith to increase the forward-bias voltage across the base-emitter junction of drive transistor Q2. The latter thereby conducts a large collector current. The resulting large voltage drop across resistor R8 causes a large forward-bias across the base-emitter junction of lower output transistor Q4 to cause the latter to conduct a large collector current.

The currents thus conducted by transistors Q2 and Q4 flow from the positive terminal of power supply B1 through said transistors to output terminal 01, through emitter resistor R12, and then through the short-circuited output transistor Q5 to the negative terminal of power supply Bl. As a result a large portion of the short-circuit current flowing through output transistor Q5 is supplied by transistors Q2 and Q4 so as to be effectively bypassed around loudspeaker S instead of flowing therethroug'h. The voice-coil of loudspeaker S is thereby subjected to a substantially smaller surge of current so as to obviate the damage that would otherwise occur.

The usual drive transistor types employed will act as protective fuses by undergoing almost instantaneous second breakdown in response to breakdown of the output transistor in the opposite half of the push-pull arrangement. That is, instead of merely passing a large collector current, transistor Q2 will instantaneously fail and become a short-circuit thereby more effectively bypassing the short-circuit current around loudspeaker S and also serving to protect the lower output transistor Q4 to prevent the latter from undergoing breakdown.

If output transistor Q4 should break down and undergo a short-circuit failure, instead of output transistor Q5, then the other drive stage transistor Q3 will provide current bypass protection for loudspeaker S in a similar manner. The direct-current coupling to the base of tran sistor Q3 and the direct-current feedback network to the base of transistor Q1 tend to maintain the potential of the base of transistor Q3 against displacement as the emitter potential of transistor Q3 is drawn towards the positive potential of power supply B1 due to the shortcircuit of output transistor Q4. The forward-bias voltage across the base-emitter junction of transistor Q3 is thereby increased and a large emitter current flows through transistor Q3 to drive output transistor Q5 heavily into the active region. A large portion of the short-circuit current flowing through transistor Q4 is thus supplied by transistors Q3 and Q5 so as to reduce substantially the surge of current through loudspeaker S. If drive transistor Q3 is of the type usually selected for this function it is likely to undergo almost instantaneous second breakdown so as to improve the efficacy of this protective arrangement.

It will be understood that drive transistors Q2 and Q3, although in a sense operate as fuses particularly when they undergo second breakdown, provide a protective function which could not be rendered even by the fastest acting instrument fuses available. It is well recognized that fuses are ineffective to protect the loudspeaker because the latter is usually damaged before the finite time required for the fuse to melt. However, the time required for drive transistors Q2 and Q3 to undergo second breakdown is so small as to result in practically instantaneous protection for the loudspeaker.

In the embodiments of FIGS. 1 to 4, 8 and 9 the protective function of the drive stage is achieved by conductively coupling the drive transistor emitters to the output terminal through bias resistors, whereas in the embodiments of FIGS. 1a, 2a, 3a and 4a the drive stage emitters are conductively coupled to the output terminal through the base-emitter junctions of the respective output transistors, as will be described below.

Referring now to FIG. 1a there is disclosed another embodiment of the invention utilizing the same principles described above with respect to FIGS. 1 to 4. Input terminal I1 is coupled by capacitor C1 to the base of PNP transistor Q1 operating Class A in the common-emitter mode. The other input terminal I2 is grounded. In this embodiment the ground, instead of a zener diode, serves as a source of reference potential in that bias resistor R1 extends from ground to the base of transistor Q1 to maintain the base at a fixed quiescent reference potential substantially independent of power supply regulation and ripple, ambient and operating temperature variations, and other variables.

A resistor R2 and capacitor C2 are connected in series between the emitter of transistor Q1 and ground. Also connected to the emitter of transistor Q1 is one end of a resistor R5 having its other end connected to output bus 0 of the amplifier. A fixed resistor R6 and a variable resistor in the form of a potentiometer P are connected in series between the B+ power supply terminal and output bus 0. A resistor R6 extends from output bus 0 to the negative terminal B- of the power supply. A resistor R4 extends between the junction of resistor R6 and potentiometer P and the junction of capacitor C2 and resistor R2.

The function of resistors R4, R6 and potentiometer 'P is to bias the emitter of transistor Q1 at a quiescent potential which is positive with respect to ground. This is necessary because of the voltage drops across the baseemitter junction of transistor Q1 and the bias resistor R1. It will be obvious that if the quiescent potential of the base is displaced in a predetermined direction and magnitude with respect to ground, as compared with the above-described embodiments, then the quiescent potential of the emitter must be identically displaced in order to maintain the same bias voltage across the base-emitter junction of the transistor.

The second stage comprises an NPN transistor Q2 operating Class A in the common-emitter mode and having its base direct-coupled to the collector of transistor Q1, its emitter A;C.-c0upled to ground through the negative supply terminal B, and its collector connected to the lower end of a first load resistor R8 having its upper end connected to a second load resistor R7 in turn connected to the positive supply terminal B+. A capacitor C4 may be provided between the collector and base of transistor Q2 to provide a rollotf and phase shift at high 

